116 
ANNIVERSARY ADDRESS 
ginous plates which prevent the alteration of its shape, and we 
find here, in a nascent form, the bony sclerotic ring which is 
so useful to birds, and which I shall mentioD hereafter. In 
some large fishes the whole outer covering is converted into a 
cup of hone with two orifices, one for the cornea, and the other 
for the entrance of the optic nerve. The fish has no lachrymal 
glands to moisten the external surface of the eye, nor indeed does 
the medium in which it lives render this necessary. The lens is 
large in proportion to the size of the eye, and is almost spherical 
and of extreme density. All these adjustments are seen to he 
required when we consider the habitat of the fish, hut they are 
not well suited to aerial vision. It is necessary that the lens should 
he of greater density than the surrounding medium, in order that 
the rays of light should be refracted towards a focus. The large 
size of the lens increases the luminosity of the image, and its 
spherical form is the most suitable owing to the high refractive 
index of the water, and for this reason divers wear strong convex 
glasses, which enable them to see distinctly. The protrusion of the 
lens through the iris enables the fish to see objects floating above 
it. Nor are these the only peculiarities in the eye of the fish. A 
delicate fold* 4 ( processus falciformis) from one of the coats of the 
capsule of the eye ( the choroid ) traverses the retina, and terminates 
in a vascular enlargement ( campanula Salieri) attached to the 
centre and back part of the capsule of the lens. Another vascular 
body of the choroid ( the choroid gland) causes the retina to project 
near the entrance of the optic nerve into the chamber of the eye¬ 
ball. This chamber is filled, as in all eyes of Vertebrata, with a 
semi-liquid ( the vitreous humour) and is closed in front by the lens. 
Cuvier and Yalenciennesf thought that the choroid gland might 
serve to adjust the eye to different distances. "When charged with 
blood it certainly may push the retina nearer to the lens. The 
campanula Salieri may also act as an accommodation, not only 
by means of the muscular fibres it contains, hut also when 
enlarged, for it may then cause the vitreous humour to alter the 
shape of the optic chamber, and thus affect the distance between 
the retina and lens. Eut the exact function of these bodies is 
not known. 
Most kinds of fishes are without eyelids, hut the sharks and 
dogfishes not only possess these organs, hut some of them have 
also a movable nictitating membrane, such as is found in many 
birds, and to which I shall refer hereafter. 
* See ‘ Die Sehorgane der Thiere,’ by Carriere, Munich, 1885. 
t ‘Histoire naturelle des Poissons,’ tome i, Paris, 1828. 
